1. Field of the Invention
The present invention relates to an MMIC (Monolithic Microwave Integrated Circuit) having a wave guide for high-frequency signal transmission.
2. Description of the Related Art
In contrast to conventional silicon integrated circuits, MMICs comprising high-speed semiconductor devices such as that represented by HEMT (High Electron Mobility Transistor) or HBT (Hetero Bipolar Transistor) necessarily include a wave guide as the inner transmission line for high-frequency signals. Micro-strip lines are generally used as the high-frequency signal wave guide, because of their stable line characteristics and low dispersion characteristics which means that the frequency dependency of the propagation constant is weak.
FIG. 1 shows an MMIC having conventional micro-strip lines, in particular, a three-dimensional MMIC having micro-strip lines composed of multi-layered line conductors.
As shown in FIG. 1, the MIMIC having a conventional multi-layered structure includes ground plate 3 formed on the semiconductor substrate 1 with the insertion of surface insulation film 2 therebetween, and ground plate 3 forms micro-strip lines together with line conductors 5 each formed on each of interlayer insulation films 4, respectively. In addition to line conductors 5, a pad 6 for the external connection is provided on the most upper interlayer insulation film 4.
The MMIC having multi-layered high-frequency micro-strip lines, as explained with reference to FIG. 1, has features that are suited to high density integration, compared to MMICs having line conductors disposed in a single layer.
However, none of reports appear to discuss the reliability of such multi-layered high-frequency micro-strip lines so far.
It is an object of the present invention to provide a structure of a three-dimensional MMIC designed by taking reliability into consideration.
In three-dimensional MMICs, the interlayer insulation films are composed of a resin (organic) insulating material such as polyimide or benzocyclobutene (BCB).
The interlayer insulation film of such resin insulating material is relatively soft and is apt to deform when pressure is applied thereto. Pads, for instance, are subjected to a mechanical shock by the tip of a bonding tool during wire bonding thereto, and deformation is caused in the interlayer insulation film around there.
As a result, line conductor 5 on the most upper interlayer insulation film tends to peel off or bend. When wire bonding is over and application of the pressure by the bonding tool is removed, the interlayer insulation film can recover from the deformation by itself. However, the line conductor that is once peeled off or bent cannot be restored, and results in the change of its high-frequency transmission characteristics.
In the present invention, a groove is provided in the interlayer insulation film adjacent to the pad, for relaxing the influence of the deformation given during wire bonding processes.
FIG. 2 shows the essential concept of the present invention. As shown in the drawing, groove 7 is provided adjacent to pad 6, and thus, the PAD REGION and the WIRING REGION are physically separated from each other at least by the groove near the respective surfaces thereof. Accordingly, even when pad 6 is subjected to wire bonding processes, aforesaid deformation caused in the interlayer insulation film by the pressure applied to pad 6 is relaxed by the shape effect of groove 7, and the influence of the deformation on the WIRING REGION can be alleviated.